Loss of Sexual Instinct in Humans about 200,000 years ago was THE defining & dinstinguishing feature of fully modern humans. Thereafter, heterosexuality ceased being innate, while homosexuality became a natural – though not inborn – tendency that societies perpetually needed to control.

Is homosexuality caused by nature, or nurture, or a combination of both? I will show by rational arguments that homosexuality can only be a function of nurture (more specifically, individual experience), and not nature, nor a combination of nature and nurture.
Let us first consider the possibility that homosexuality is caused by nature, or genes, in other words. This proposition basically says that that there is an actively expressed gene in homosexuals that is not found (or not expressed) in heterosexuals, which irresistibly causes the development of a homosexual orientation.
No such gene has ever been found, despite innumerable scientific investigations. Even the American Psychiatric Association reversed their position on this in 2008, stating that there is no compelling scientific evidence that would allow scientists to conclude that genes are responsible for homosexuality. To be fair, not finding such a gene does not, on its own, rule out it nonetheless might exist. However, there is a damning argument against the existence of such a gene: it could never survive in the gene pool because homosexuals would not pass on this gay gene to a subsequent generation.
But then, what about the recent suggestion that kin selection might allow a homosexual gene to survive, despite its reproductive shortcomings? Kin selection basically says that evolution can sometimes favour the reproductive success of an organism’s relatives, even at the cost of its own survival. Because close relatives often share some genes, a gene that would otherwise be weeded out by natural selection can survive if it is carried by an organism’s relatives. In the case of homosexuality, it has been suggested that homosexuals, by virtue of them not marrying, have more time and resources to devote in the rearing of their relative’s children, thereby increasing their chances of survival.
However seductive this argument might appear, it is completely invalid. The reason is that if homosexuals did indeed have the tendency to contribute to increasing the survival chances of their relative’s offspring in any significant way, then natural selection would have increased the presence of the homosexual gene to the extent that heterosexuals everywhere would have access to such additional familial care and resources from a homosexual relative. Yet all the statistics on homosexuality show that at most, 10% of human beings are homosexual, a number too small to service the familial needs of the remaining 90% of the population. Evolution relentessly maximizes reproductive fitness, and given that it has not optimized the prevalence of homosexuality in humans to this end, it can only be interpreted as proving that kin selection for the homosexual gene is non-existent. Besides, if homosexuals in society had served this beneficial role since time immemorial, one would think it would long ago have become part of humanity’s collective knowledge and defined universally favourable attitudes toward them. We would not have needed a scientist to come up with such a theory.If a homosexual gene does not exist, then it follows that a combination of nature and nurture can also be ruled out as causing homosexuality. Why? Because if any gene existed that could be acted upon by the environment in such a way as to produce homosexuality, then that gene would be eliminated by natural selection for the same aforesaid reasons. Therefore, the only conclusion one can draw is that homosexuality must have a completely non-genetic basis: if it’s not due to nature, and it’s not due to a combination of nature and nurture, then it must be due totally to nurture. The same reasoning must apply to other animal species as well: homosexual activity everywhere in the animal kingdom must be due only to environment, and cannot have a genetic basis.